1. Field of the Invention
The present invention relates to a method for processing video signals in a digital image transformer and more particularly to a processing method for improving the quality of the data displayed on a television monitor controlled by an image transformer. This data may be radar data. It may also be data coming from a sonar, infrared sensors, echography systems, which data it is desirable to display on screens operating in a television mode. The invention also relates to a device for implementing such a method.
2. Description of the Prior Art
In the prior art, image transformers used memory tubes generally comprising two guns, a writing gun controlled with radar scanning and a reading gun controlled by television scanning. Then digital image transformers, which use digital circuits, appeared.
FIG. 1 gives a schematic representation of a digital image transformer (DIT) to which the present invention applies. The DIT comprises a circuit 1, called radar interface, which receives the radar video signals VI together with the sync signals VY, and a circuit 2 for converting the coordinates, P, .theta., of the radar video into XY coordinates for cartesian representation on the television screen. Circuits 1 and 2 are connected to a digital memory 3, which is a random access memory or RAM. A so-called persistance circuit 4 is connected between radar interface circuit 1 and memory 3. Circuit 4 is connected to a display circuit 5 via memory 3. The television display 6 is connected to circuit 5.
The functions of the different circuits of the DIT are as follows. Interface circuit 1 samples and converts into digital the analog video signals, which are applied thereto. Interface circuit 1 may comprise a video compression circuit for the real time acquisition of a radar radial (radar scan), i.e. the acquisition of the video signals received by the radar after emission thereby of a sync pulse for a definite angle of the rotating antenna with respect to a given origin, and the reading of these video signals, with a time lag and at a different speed, so as to match the access times into image memory 3. Circuit 2 converts the signals from polar coordinates into cartesian coordinates, thereby allowing the address of each image element in cartesian coordinates to be calculated from the radar information received in the form of polar coordinates. Image memory 3 has a resolution adapted to the television standard used. It may, for example, use 1024 memory cells for 1024 lines. Hence, an image point to be displayed corresponds to each cell. The luminance of each point may be coded, for example, by means of three bits, thus generating eight luminosity levels for each point. For this memory, the television reading and radar writing phases are asynchronous. The reading is privileged and during a reading phase, the conversion is stopped. Display circuit 5 provides for the generation of television sync signals and the simultaneous reading of several points in the image memory. The latter operation permits the display circuit to comply with the access times of the circuits used and to allow writing of the points into this same memory. Display circuit 5 further converts the digital luminosity data read from the image memory into an analog format for generating an analog video-television signal intended for the associated television monitor on which the displayed data appears. The purpose of persistance circuit 4 is to restitute, for data given in digital form for which the persistance does not exist, a persistance effect comparable to that which is produced on a memory tube. On a memory tube, in fact, the brightness of a dot begins to decrease as soon as it has been written. The persistance circuit creates a similar effect, but with a delay of one antenna revolution and a quantified level decay at each revolution.
In a digital image transformer, the radar data displayed must have a certain quality which results in certain constraints to be complied with during coordinate conversion and storing. The insufficiency of such quality causes a fluctuation of the displayed image.
In fact, writing of the radar video into the memory after conversion of the coordinates must allow, for the persistance to be introduced, correct ageing of the stored data. To comply with a pre-established persistance law, it is necessary to address the same memory cells from one antenna revolution to the next, i.e. the required accuracy of the display requires that a point of a video signal, situated at a distance P from the origin of the coordinates for an angle .theta. of rotation of the antenna, be found, after conversion, always in the same memory cell. Addressing a point of the memory is given by the following relationship: EQU X=P cos .theta. EQU Y=P sin .theta.
Two sources of inaccuracies should then be noted, one relating to the distance vector radius P, the other to the angle .theta. of rotation of the antenna.
The error in the determination of P is due to a shift in the phase of the sampling clock of the video with respect to the synchronization of the radar. The error in determining the angle .theta. is due to the way in which the beginning of the conversion is made. This lack of accuracy in determining P and .theta. causes errors in the telemetry measurements.
Moreover, it will be noted that writing of the video into the memory after conversion of the coordinates must be carried out so as to mininize the number of radials, otherwise known as radar scans, converted per antenna revolution. The limitation of the number of the radials converted to the number of angle increments per revolution allows a corresponding increase in the speed of rotation of the antenna of the radar permissible by the image transformer.
To obtain such a quality of the displayed images and such an accuracy in determining the distance and the angle of rotation of the antenna, some means have been proposed in the prior art.
It has for example been proposed to use a free oscillator for sampling the video, but this results in an error being introduced into the distance.
It has been proposed, for solving the problem of persistance and the number of converted radials, to effect the conversions on the angle increments. For this purpose, the video is written into one memory out of two. At each angle increment the memories are switched and the data contained in the other memory is converted. In this method, however, for a given angle, there is a risk of converting video signals belonging to several adjacent radials. In fact, in general, the emission of the sync pulse of the radar and the rotation of its antenna are asynchronous from one antenna revolution to the next for a given angle. Thus, the pieces of converted radials are not the same. This causes a loss of data when the repetition frequency is greater than the resolution of the antenna coder.
For a point of an electronic card generated by polar scanning, the angle .theta. error is one angle increment (.DELTA..theta.). A video point on such a card is calculated from the angle value at the time of emission of the sync pulse. An angle increment error causes consequently a positioning error at the level of the memory cells. The maximum error is obtained by the greatest off-centering, and the error distance D is given by D=P.DELTA..theta. and Dmax=(.DELTA..theta.). (Off-centering is defined as the distance between the center of the radar given by the antenna and the center of the display screen chosen by the operator.) For a transformer having as definition 1024 lines.times.1024 points, it can be calculated that the angle error can reach four points and the distance error one point. This makes the quality of the display of a card generated with polar coordinates poor and it reduces the accuracy of the telemetry measurements which are effected.